This invention relates generally to a method, apparatus, and system for optimizing transmission power and bit rate in multi-transmission scheme communication systems. More specifically, this invention relates to a method, apparatus, and system for optimizing transmission power and bit rate in multi-transmission scheme communication systems that employ variable modulation schemes and/or coding rates.
Many of today's communication systems use multiple transmission schemes to exchange network information. One such system, popular in today's cellular networks, is the Global System for Mobile Communications transmission scheme, or GSM. GSM uses a Time Division Multiple Access (TDMA) access scheme. The basic radio resource in a GSM system is a time slot, lasting approximately 577 μs, and transmitting information at a modulation rate of approximately 271 Kbit/s. Each time slot carries a slice of information approximately 156 symbols (577 μs*271 Kbit/s) in length, known as a burst. GSM time slots are grouped into TDMA frames, each frame comprising eight time slots. TDMA frames are grouped into either 26-frame or 51-frame multiframes. The TDMA multiframes are then grouped into a TDMA superframe, which in turn are grouped to form a TDMA hyperframe.
GSM systems transmit and receive information over a band of radio frequency (RF) carriers, typically around 900 MHZ. The frequency separation between carriers in an RF band is specified to be 200 KHz. Each GSM carrier comprises eight basic physical channels. Information carried over these channels switches carriers, or “hops” between carriers, several times according to a predefined sequence when being transferred from/to mobile stations (MS) to/from base stations (BS) operating in the network. The frequency hopping sequences are chosen to be orthogonal within a cell to maximize use of the available spectrum in the RF band. Thus, a GSM physical channel is defined as a sequence of TDMA frames, a time slot number (modulo 8), and a frequency hopping sequence.
Before being modulated over the various physical channels, digitally sampled speech and data is first coded by a channel codec to arrange the information into the final form necessary for RF transmission. Channel coding involves adding additional data for channel control, training sequences, and tail/guard bits. In addition, the channel coder must interleave the data to enhance the performance of the error correction and to rearrange the data into packets for transmissions. Training sequence data is added for equalization of the RF channel, while tail/guard bits provide a buffer between adjacent data packets. The coding process represents a signaling overhead that adds a significant number of bits of information to the burst, thus reducing the overall information-carrying capacity and net throughput of the physical channels.
After being coded and packetized, information bursts are ready to be modulated onto the carriers for transmission to or from the MS or BS. GSM uses a type of modulation known as shift keying, which is particularly suited for modulating digital signals. At the time of its inception, GSM used only one type of modulation, known as Gaussian Minimum Shift Keying (or GMSK). In this type of modulation, the phase of the modulated signal is rotated along the unit circle in increments of π/2. Increasing phase represents one bit value (perhaps a “1”), while decreasing phase represents the other bit value (perhaps a “0”). Thus each symbol represents one bit of information in the modulated signal. As indicated by its name, GMSK uses a Gaussian bandpass filter to filter the modulated signal prior to being transmitted on the carrier. The resultant filtered waveform has only a minimal amount of inter-symbol interference (ISI), yet has an improved power spectral density over other phase shift keying modulation schemes.
GSM capability was expanded with the adoption of the Enhanced General Purpose Radio System (EGPRS) standard in 2000. This standard increased the maximum available data rate for packet traffic and control channels by expanding the number of coding schemes, and adopting a new modulation scheme, Eight-Phase Phase Shift Keying (8PSK). The eight phases of the modulated waveform represent three bits (modulo 3) of information, thus each symbol in the modulated signal is capable of representing three times the amount of information as a corresponding symbol in a GMSK modulated waveform. The EGPRS standard resulted in the adoption of nine coding/modulation schemes, MCS-1 through MCS-9, the parameters of which are summarized in Table 1 below.
TABLE 1Coding parameters for the EGPRS coding schemesCODING RATETRANSMISSION(User Data/MODULATIONDATA RATESCHEMEXMIT Data)TYPEKbit/sMCS-91.08-PSK59.2MCS-80.9254.4MCS-70.7644.8MCS-60.4929.6/27.2MCS-50.3722.4MCS-41.0GMSK17.6MCS-30.8014.8/13.6MCS-20.6611.2MCS-10.53 8.8
As Table 1 indicates, the maximum data rate is determined by both the coding rate (i.e., the ratio of user data to transmitted data) and the modulation scheme employed. For example, MCS-1 is the most robust of the nine transmission schemes, as this scheme introduces the highest overall number of coded bits into the data transmission (47% of the data transmitted is error coding information). MCS-1, however, provides the lowest data rate, as the highly coded data stream is transmitted using the lower bit-rate GMSK modulation scheme. In contrast, MCS-9 yields the highest bit rate of the nine modulation schemes by introducing a limited amount of coded bits into the data transmission thus minimizing overhead, and by using the higher bit-rate 8PSK modulation scheme. Because of the minimal of amount coding introduced in the data transmission, however, MCS-9 is the least robust of the nine transmission schemes. Higher degrees of robustness are required as the amount of interferers in a cellular region increases, or as the amount of available transmission power or the amount of sensitivity in the BS or MS decreases. The remaining transmission schemes shown in Table 1 offer various tradeoffs between data rate and transmission robustness.
The conventional approach for establishing a connection between terminals operating in a cellular region is shown in FIG. 1. After selecting a transmit power level (step 101), the BS selects the most aggressive transmission scheme (e.g., MCS-9) to exchange information with other terminals in the cell at step 103. Next, the link performance is measured at step 105 using the selected transmit power and transmission scheme. If it is determined that the link performance is acceptable at step 107, then slices of information are exchanged between the terminals at step 111 at a time slot rate (or slice rate) that ensures that the transmitting device(s) does not overheat. If, however, it is determined at step 107 that the link performance is not acceptable, a more robust transmission scheme (e.g., MCS-8) is selected at step 109, and the link performance is again evaluated at step 105. The transmission scheme selection process (i.e. steps 105–109) repeats until acceptable link performance is obtained. Once a reliable transmission scheme is selected, information is exchanged between the terminals at step 111 at a time slot rate that again ensures that the transmitting device(s) does not overheat.
While this conventional approach results in the highest available burst transfer rate being selected for the connection, it is not the most efficient method for maximizing the net data rate of a channel. The net data rate of a channel is not only affected by the channel burst transfer rate, but is also determined by the number of time slots that can be used to exchange data over a given period of time.
Cellular network regions (or cells) often operate at less than peak capacity. For example, in more rurally located cells, the available bandwidth is often such that a single user could exchange information in multiple time slots of the same TDMA frame without significantly affecting the overall performance of the cellular network. When these conditions exist, it would be advantageous to occupy as many time slots with information as possible in a given period in order to maximize the net data transfer rate. There exist, however, several factors that limit the number of time slots per given period that can be used to exchange information between a MS and BS in given cell.
An important limiting factor in determining the number of time slots available for data transfer is the amount of heat that is generated in the MS transmitter as a result of the data transmission. The amount of heat generated is directly proportional to the transmission power level of the MS, and to the number of time slots used to transmit the data or voice information. The more time slots used in a given period to transmit information, or the higher the transmission power level of the MS, the greater the amount of heat generated in the transmitter. To avoid overheating in the MS, the rate of time slot usage must be reduced, thus reducing the net data transfer rate of the device. In practice, conventional handsets have significantly limited multi-slot capability due to overheating concerns, which necessary limits the amount of bandwidth the handsets can utilize.